The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI) scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged or examined is placed. Via magnetic resonance radio frequency (RF) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped (via RF pulses) into a plane transverse to the main magnetic field where they precess or resonate. In turn, the resonating dipoles are allowed to decay or realign with the main magnetic field while inducing detectable magnetic resonance (MR) echoes from a selected region of the subject. The various echoes making up the MRI signal are typically encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix commonly known as k-space. Ultimately, in an imaging experiment, by employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space data.
The commonly known echo planar imaging (EPI) is a rapid MRI technique which is used to produce tomographic images at high acquisition rates, typically several images per second. It has been found useful in perfusion and/or diffusion studies, for functional magnetic resonance imaging (fMRI), in dynamic-contrast studies, etc. However, images obtained in EPI experiments tend to be vulnerable to an artifact know as xe2x80x9cghostingxe2x80x9d or xe2x80x9cghost images.xe2x80x9d The xe2x80x9cghostsxe2x80x9d are typically positioned at N/2 pixels relative to the true or desired object image position (where N is the number of pixels across the image field of view (FOV)). More specifically, alternating errors or cyclic errors can be generated in the k-space data due to common system limitations or imperfections such as, e.g., imperfect gradient application, non-linear system responses (i.e., Maxwell fields, mechanical displacements or vibrations, etc.), instabilities in digital to analog conversion timing, or inherent properties of the imaged object (i.e., susceptibility differences, flow/respiratory changes, chemical shifts, etc.). The cyclic errors are typically created by differences in the odd and even horizontal data lines of k-space, e.g., misalignment of the data line peaks, or phase shift errors. These may be denoted as cyclic errors, because each full cycle of the readout gradient contains both a positive polarity portion and a negative polarity portion, and within the full cycle there is mismatch or error between the two polarity portions. The same error is largely repeated in each successive cycle. Likewise, these errors may be denoted as alternating, because each cycle produces a pair of consecutive data lines, with the odd numbered lines exhibiting substantially consistent data, the even numbered lines also being substantially consistent, but the neighboring even and odd lines exhibiting relative error or inconsistency. In any event, the Fourier reconstruction tends to convert the cyclic errors into secondary images or xe2x80x9cghostsxe2x80x9d that are shifted by a half-image from the primary or true desired image of the object.
The ghost images can obscure the true desired image, reduce image clarity or sharpness, and generally degrade overall image quality. Moreover, high levels of ghosting can produce false readings that lead to diagnostic error. Accordingly, it is highly desirable to produce EPI images that are essentially free of ghost artifacts.
However, previously developed techniques for addressing ghosting in EPI experiments remain subject to certain drawbacks or limitations. For example, one popular and well know method for EPI ghost reduction is the phase reference scan which employs a reference scan with zero phase encoding prior to the imaging pulse sequence. By examining offsets in the echo between even and odd echo acquisitions, a set of phase correction values is determined. The goal of the phase reference scan technique has typically been to remove zero and first order phase differences between odd and even echoes, and has been shown capable of reducing an amount of ghosting. Still, the phase reference scan technique is known to occasionally increase the N/2 ghosting artifact. Reference scan methods may introduce error into images if there is deviation or inconsistency between the reference acquisition and the associated image acquisition, or if there are flawed results generated from the analysis of the reference image.
Another method in the prior art involves the collection of EPI raw data in which a data line or small number of data lines are replicated. Time shifts and perhaps phase shifts can then be estimated by looking at the location and phases of maximal signal in each line of the free induction decay (FID) readout. However, this technique is disadvantageous insomuch as the additional data lines disrupt the continuous readout in the phase encode direction and introduces point spread errors for signals not on resonance. Additionally, estimating phase differences between alternating data lines with only two data lines results in an inability to discern the alternating part of the signal variation from gradual linear (non-alternating) drifts which cause peak misalignment.
Moreover, many previously developed techniques are relatively complex and time intensive. Additionally, some of them require active operator intervention and/or judgment to effect ghost reduction, thereby putting demands on the operator""s time and leaving open the possibility of operator error.
The present invention contemplates a new and improved technique for reducing ghosting in EPI images which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging includes supporting a subject in an examination region of an MRI scanner, and applying an EPI pulse sequence with the MRI scanner to induce a detectable magnetic resonance signal from a selected region of the subject. The magnetic resonance signal are received and demodulated to generate raw data. Applied to the raw data are a pair of ghost reducing correction factors. The pair of corrections factors include a phase correction and a read delay. The phase correction compensates for phase errors in the raw data, and the read delay effectively shifts a data acquisition window under which the raw data was collected to thereby align the raw data in k-space. The correction factors affect how data is loaded into k-space to generate k-space data, and the k-space data is subjected to a reconstruction algorithm to generate image data. Thereafter, values for the pair of correction factors are derived from the image data.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging is provided. It includes supporting a subject in an examination region of an MRI scanner, and conducting an iterative calibration procedure. The iterative calibration procedure includes applying a calibration EPI pulse sequence with the MRI scanner to induce a detectable magnetic resonance signal from a selected region of the subject. The calibration EPI pulse sequence has phase encoding gradient pulses of a first amplitude. The magnetic resonance signal is received and demodulating to generate raw data to which is applied a pair of ghost reducing correction factors. The pair of corrections factors includes a phase correction and a read delay. The phase correction compensates for phase errors in the raw data, and the read delay effectively provides a timing shift to be applied to the raw data to thereby align the raw data in k-space. The correction factors affect how data is loaded into k-space to generate k-space data which is subjected to a reconstruction algorithm to generate image data. Values for the pair of correction factors are derived from the image data such that with each ensuing repetition of the calibration procedure the derived values are used for the pair of correction factors applied in the following repetition. Upon completion of the iterative calibration procedure, the last derived values are designated for use as the correction factors in a subsequent imaging experiment directed to the selected region of the subject.
In accordance with another aspect of the present invention, an MRI scanner includes a main magnet that generates a substantially uniform temporally constant main magnetic field through an examination region wherein an object being imaged is positioned. A magnetic gradient generator produces magnetic gradients in the main magnetic field across the examination region, and a transmission system includes an RF transmitter that drives an RF coil which is proximate to the examination region. A sequence control manipulates the magnetic gradient generator and the transmission system to produce an EPI pulse sequence. The EPI pulse sequence induces a detectable magnetic resonance signal from the object, and a reception system, which includes a receiver, receives and demodulates the magnetic resonance signal to obtain raw data which is loaded into a k-space storage device. A reconstruction processor subjects the k-space data to a reconstruction algorithm to generate images data which is loaded into an image data storage device. An output device ultimately produces human-viewable image representations of the object from the image data. In addition, an automatic ghost reducing means automatically generates from the image data, correction factors that are applied to the raw data for reducing ghosting in the image representations.
One advantage of the present invention is consistent reliable automated ghost reduction in EPI experiments including higher order corrections.
Another advantage of the present invention is improved image quality.
Yet another advantage of the present invention is reduced calibration time through the use of interpolated correction factors for ghost reduction.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.